<html>
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    <title>Resumable C++ 20 Coroutines</title>
    <link rel="stylesheet" href="../../../boostbook.css" type="text/css">
    <meta name="generator" content="DocBook XSL Stylesheets V1.75.2">
    <link rel="home" href="../../../index.html" title="Asio">
    <link rel="up" href="../composition.html" title="Composition and Completion Tokens">
    <link rel="prev" href="cpp20_coroutines.html" title="C++20 Coroutines Support">
    <link rel="next" href="../networking.html" title="Networking">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
<table cellpadding="2" width="100%">
    <tr>
        <td valign="top"><img alt="asio C++ library" width="250" height="60" src="../../../asio.png"></td>
    </tr>
</table>
<hr>
<div class="spirit-nav">
    <a accesskey="p" href="cpp20_coroutines.html"><img src="../../../prev.png" alt="Prev"></a><a accesskey="u"
                                                                                                 href="../composition.html"><img
        src="../../../up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../home.png"
                                                                                            alt="Home"></a><a
        accesskey="n" href="../networking.html"><img src="../../../next.png" alt="Next"></a>
</div>
<div class="section">
    <div class="titlepage">
        <div>
            <div><h4 class="title">
                <a name="asio.overview.composition.coro"></a><a class="link" href="coro.html"
                                                                title="Resumable C++ 20 Coroutines">Resumable C++ 20
                Coroutines</a>
            </h4></div>
        </div>
    </div>
    <div class="note">
        <table border="0" summary="Note">
            <tr>
                <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../note.png"></td>
                <th align="left">Note</th>
            </tr>
            <tr>
                <td align="left" valign="top"><p>
                    This is an experimental feature.
                </p></td>
            </tr>
        </table>
    </div>
    <p>
        The <a class="link" href="../../reference/experimental__coro.html" title="experimental::coro"><code
            class="computeroutput"><span class="identifier">experimental</span><span class="special">::</span><span
            class="identifier">coro</span></code></a>
        class provides support for a universal C++20 coroutine. These coroutines
        can be used as tasks, generators and transfomers, depending on their signature.
    </p>
    <pre class="programlisting"><span class="identifier">coro</span><span class="special">&lt;</span><span
            class="identifier">std</span><span class="special">::</span><span class="identifier">string_view</span><span
            class="special">&gt;</span> <span class="identifier">line_reader</span><span class="special">(</span><span
            class="identifier">tcp</span><span class="special">::</span><span class="identifier">socket</span> <span
            class="identifier">stream</span><span class="special">)</span>
<span class="special">{</span>
   <span class="keyword">while</span> <span class="special">(</span><span class="identifier">stream</span><span
                class="special">.</span><span class="identifier">is_open</span><span class="special">())</span>
   <span class="special">{</span>
     <span class="identifier">std</span><span class="special">::</span><span class="identifier">array</span><span
                class="special">&lt;</span><span class="keyword">char</span><span class="special">,</span> <span
                class="number">4096</span><span class="special">&gt;</span> <span class="identifier">buf</span><span
                class="special">;</span>

     <span class="keyword">auto</span> <span class="identifier">read</span> <span class="special">=</span> <span
                class="identifier">co_await</span> <span class="identifier">stream</span><span
                class="special">.</span><span class="identifier">async_read_some</span><span class="special">(</span>
         <span class="identifier">asio</span><span class="special">::</span><span class="identifier">buffer</span><span
                class="special">(</span><span class="identifier">buf</span><span class="special">),</span> <span
                class="identifier">experimental</span><span class="special">::</span><span
                class="identifier">use_coro</span><span class="special">);</span>

     <span class="keyword">if</span> <span class="special">(</span><span class="identifier">read</span> <span
                class="special">==</span> <span class="number">0u</span><span class="special">)</span>
       <span class="keyword">continue</span><span class="special">;</span>

     <span class="identifier">co_yield</span> <span class="identifier">std</span><span class="special">::</span><span
                class="identifier">string_view</span> <span class="special">{</span> <span class="identifier">buf</span><span
                class="special">.</span><span class="identifier">data</span><span class="special">(),</span> <span
                class="identifier">read</span> <span class="special">};</span>
   <span class="special">}</span>
<span class="special">}</span>

<span class="identifier">coro</span><span class="special">&lt;</span><span class="keyword">void</span><span
                class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span
                class="identifier">size_t</span><span class="special">&gt;</span> <span
                class="identifier">line_logger</span><span class="special">(</span><span
                class="identifier">tcp</span><span class="special">::</span><span class="identifier">socket</span> <span
                class="identifier">stream</span><span class="special">)</span>
<span class="special">{</span>
  <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span
                class="identifier">lines_read</span> <span class="special">=</span> <span class="number">0u</span><span
                class="special">;</span>
  <span class="keyword">auto</span> <span class="identifier">reader</span> <span class="special">=</span> <span
                class="identifier">line_reader</span><span class="special">(</span><span
                class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span
                class="special">(</span><span class="identifier">stream</span><span class="special">));</span>
  <span class="keyword">while</span> <span class="special">(</span><span class="keyword">auto</span> <span
                class="identifier">l</span> <span class="special">=</span> <span
                class="identifier">co_await</span> <span class="identifier">reader</span><span class="special">)</span>
  <span class="special">{</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span
                class="special">&lt;&lt;</span> <span class="string">"Read: '"</span> <span
                class="special">&lt;&lt;</span> <span class="special">*</span><span class="identifier">l</span> <span
                class="special">&lt;&lt;</span> <span class="string">"'"</span> <span
                class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span
                class="identifier">endl</span><span class="special">;</span>
    <span class="identifier">lines_read</span><span class="special">++;</span>
  <span class="special">}</span>
  <span class="identifier">co_return</span> <span class="identifier">lines_read</span><span class="special">;</span>
<span class="special">}</span>

<span class="keyword">void</span> <span class="identifier">read_lines</span><span class="special">(</span><span
                class="identifier">tcp</span><span class="special">::</span><span class="identifier">socket</span> <span
                class="identifier">sock</span><span class="special">)</span>
<span class="special">{</span>
  <span class="identifier">co_spawn</span><span class="special">(</span><span class="identifier">line_logger</span><span
                class="special">(</span><span class="identifier">std</span><span class="special">::</span><span
                class="identifier">move</span><span class="special">(</span><span class="identifier">sock</span><span
                class="special">),</span>
      <span class="special">[](</span><span class="identifier">std</span><span class="special">::</span><span
                class="identifier">exception_ptr</span><span class="special">,</span> <span
                class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span
                class="identifier">lines</span><span class="special">)</span>
      <span class="special">{</span>
        <span class="identifier">std</span><span class="special">::</span><span class="identifier">clog</span> <span
                class="special">&lt;&lt;</span> <span class="string">"Read "</span> <span
                class="special">&lt;&lt;</span> <span class="identifier">lines</span> <span
                class="special">&lt;&lt;</span> <span class="string">" lines"</span> <span
                class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span
                class="identifier">endl</span><span class="special">;</span>
      <span class="special">}));</span>
<span class="special">}</span>
</pre>
    <p>
        A <a class="link" href="../../reference/experimental__coro.html" title="experimental::coro"><code
            class="computeroutput"><span class="identifier">coro</span></code></a>
        is highly configurable, so that it can cover a set of different use cases.
    </p>
    <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span>
    <span class="keyword">typename</span> <span class="identifier">Yield</span><span class="special">,</span>
    <span class="keyword">typename</span> <span class="identifier">Return</span> <span class="special">=</span> <span
                class="keyword">void</span><span class="special">,</span>
    <span class="keyword">typename</span> <span class="identifier">Executor</span> <span class="special">=</span> <span
                class="identifier">any_io_executor</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">coro</span><span class="special">;</span>
</pre>
    <h6>
        <a name="asio.overview.composition.coro.h0"></a>
        <span><a name="asio.overview.composition.coro.yield"></a></span><a class="link"
                                                                           href="coro.html#asio.overview.composition.coro.yield">Yield</a>
    </h6>
    <p>
        The <code class="computeroutput"><span class="identifier">Yield</span></code> parameter designates
        how a <code class="computeroutput"><span class="identifier">co_yield</span></code> statement
        behaves. It can either be a type, like <code class="computeroutput"><span class="keyword">int</span></code>
        or a signature with zero or one types:
    </p>
    <pre class="programlisting"><span class="identifier">coro</span><span class="special">&lt;</span><span
            class="keyword">void</span><span class="special">&gt;</span> <span class="comment">// A coroutine with no yield</span>
<span class="identifier">coro</span><span class="special">&lt;</span><span class="keyword">int</span><span
                class="special">&gt;</span> <span class="comment">// A coroutine that can yield int</span>

<span class="identifier">coro</span><span class="special">&lt;</span><span class="keyword">void</span><span
                class="special">()&gt;</span> <span class="comment">// A coroutine with no yield</span>
<span class="identifier">coro</span><span class="special">&lt;</span><span class="keyword">int</span><span
                class="special">()&gt;</span> <span class="comment">// A coroutine that can yield int</span>

<span class="identifier">coro</span><span class="special">&lt;</span><span class="keyword">int</span><span
                class="special">(</span><span class="keyword">double</span><span class="special">)&gt;</span> <span
                class="comment">// A coroutine that can yield int and receive double</span>
</pre>
    <p>
        Receiving a value means that the <code class="computeroutput"><span class="identifier">co_yield</span></code>
        statement returns a value.
    </p>
    <pre class="programlisting"><span class="identifier">coro</span><span class="special">&lt;</span><span
            class="keyword">int</span><span class="special">(</span><span class="keyword">int</span><span
            class="special">)&gt;</span> <span class="identifier">my_sum</span><span class="special">(</span><span
            class="identifier">any_io_executor</span><span class="special">)</span>
<span class="special">{</span>
  <span class="keyword">int</span> <span class="identifier">value</span> <span class="special">=</span> <span
                class="number">0</span><span class="special">;</span>
  <span class="keyword">while</span> <span class="special">(</span><span class="keyword">true</span><span
                class="special">)</span>
    <span class="identifier">value</span> <span class="special">+=</span> <span class="identifier">co_yield</span> <span
                class="identifier">value</span><span class="special">;</span> <span
                class="comment">//sum up all values</span>
<span class="special">}</span>
</pre>
    <p>
        Putting values into a coroutine can be done it two ways: either by direct
        resumption (from another coro) or through async_resume. The first value
        gets ignored because the coroutines are lazy.
    </p>
    <pre class="programlisting"><span class="identifier">coro</span><span class="special">&lt;</span><span
            class="keyword">void</span><span class="special">&gt;</span> <span class="identifier">c</span><span
            class="special">(</span><span class="identifier">any_io_executor</span> <span class="identifier">exec</span><span
            class="special">)</span>
<span class="special">{</span>
  <span class="keyword">auto</span> <span class="identifier">sum</span> <span class="special">=</span> <span
                class="identifier">my_sum</span><span class="special">(</span><span class="identifier">exec</span><span
                class="special">);</span>
  <span class="identifier">assert</span><span class="special">(</span><span class="number">0</span>  <span
                class="special">==</span> <span class="identifier">co_await</span> <span
                class="identifier">sum</span><span class="special">(-</span><span class="number">1</span><span
                class="special">));</span>
  <span class="identifier">assert</span><span class="special">(</span><span class="number">0</span>  <span
                class="special">==</span> <span class="identifier">co_await</span> <span
                class="identifier">sum</span><span class="special">(</span><span class="number">10</span><span
                class="special">));</span>
  <span class="identifier">assert</span><span class="special">(</span><span class="number">10</span> <span
                class="special">==</span> <span class="identifier">co_await</span> <span
                class="identifier">sum</span><span class="special">(</span><span class="number">15</span><span
                class="special">));</span>
  <span class="identifier">assert</span><span class="special">(</span><span class="number">25</span> <span
                class="special">==</span> <span class="identifier">co_await</span> <span
                class="identifier">sum</span><span class="special">(</span><span class="number">0</span><span
                class="special">));</span>
<span class="special">}</span>

<span class="identifier">awaitable</span><span class="special">&lt;</span><span class="keyword">void</span><span
                class="special">&gt;</span> <span class="identifier">a</span><span class="special">()</span>
<span class="special">{</span>
  <span class="keyword">auto</span> <span class="identifier">sum</span> <span class="special">=</span> <span
                class="identifier">my_sum</span><span class="special">(</span><span
                class="identifier">co_await</span> <span class="identifier">this_coro</span><span
                class="special">::</span><span class="identifier">executor</span><span class="special">);</span>
  <span class="identifier">assert</span><span class="special">(</span><span class="number">0</span>  <span
                class="special">==</span> <span class="identifier">co_await</span> <span
                class="identifier">sum</span><span class="special">.</span><span
                class="identifier">async_resume</span><span class="special">(-</span><span class="number">1</span><span
                class="special">,</span> <span class="identifier">use_awaitable</span><span class="special">));</span>
  <span class="identifier">assert</span><span class="special">(</span><span class="number">0</span>  <span
                class="special">==</span> <span class="identifier">co_await</span> <span
                class="identifier">sum</span><span class="special">.</span><span
                class="identifier">async_resume</span><span class="special">(</span><span class="number">10</span><span
                class="special">,</span> <span class="identifier">use_awaitable</span><span class="special">));</span>
  <span class="identifier">assert</span><span class="special">(</span><span class="number">10</span> <span
                class="special">==</span> <span class="identifier">co_await</span> <span
                class="identifier">sum</span><span class="special">.</span><span
                class="identifier">async_resume</span><span class="special">(</span><span class="number">15</span><span
                class="special">,</span> <span class="identifier">use_awaitable</span><span class="special">));</span>
  <span class="identifier">assert</span><span class="special">(</span><span class="number">25</span> <span
                class="special">==</span> <span class="identifier">co_await</span> <span
                class="identifier">sum</span><span class="special">.</span><span
                class="identifier">async_resume</span><span class="special">(</span><span class="number">0</span><span
                class="special">,</span> <span class="identifier">use_awaitable</span><span class="special">));</span>
<span class="special">}</span>
</pre>
    <h6>
        <a name="asio.overview.composition.coro.h1"></a>
        <span><a name="asio.overview.composition.coro._code__phrase_role__keyword__noexcept__phrase___code_"></a></span><a
            class="link"
            href="coro.html#asio.overview.composition.coro._code__phrase_role__keyword__noexcept__phrase___code_"><code
            class="computeroutput"><span class="keyword">noexcept</span></code></a>
    </h6>
    <p>
        A coro may be noexcept:
    </p>
    <pre class="programlisting"><span class="identifier">coro</span><span class="special">&lt;</span><span
            class="keyword">void</span><span class="special">()</span> <span class="keyword">noexcept</span><span
            class="special">&gt;</span> <span class="identifier">c</span><span class="special">;</span>
<span class="identifier">coro</span><span class="special">&lt;</span><span class="keyword">int</span><span
                class="special">()</span> <span class="keyword">noexcept</span><span class="special">&gt;</span> <span
                class="identifier">c</span><span class="special">;</span>
<span class="identifier">coro</span><span class="special">&lt;</span><span class="keyword">int</span><span
                class="special">(</span><span class="keyword">double</span><span class="special">)</span> <span
                class="keyword">noexcept</span><span class="special">&gt;</span> <span class="identifier">c</span><span
                class="special">;</span>
</pre>
    <p>
        This will change its @c async_resume signature, from <code class="computeroutput"><span
            class="keyword">void</span><span class="special">(</span><span class="identifier">std</span><span
            class="special">::</span><span class="identifier">exception_ptr</span><span class="special">)</span></code>
        to <code class="computeroutput"><span class="keyword">void</span><span class="special">()</span></code> or <code
            class="computeroutput"><span class="keyword">void</span><span class="special">(</span><span
            class="identifier">std</span><span class="special">::</span><span
            class="identifier">exception_ptr</span><span class="special">,</span> <span class="identifier">T</span><span
            class="special">)</span></code>
        to <code class="computeroutput"><span class="keyword">void</span><span class="special">(</span><span
            class="identifier">T</span><span class="special">)</span></code>. A
        noexcept coro that ends with an exception will cause <code class="computeroutput"><span
            class="identifier">std</span><span class="special">::</span><span class="identifier">terminate</span></code>
        to be called.
    </p>
    <p>
        Furthermore, calls of <code class="computeroutput"><span class="identifier">async_resume</span></code>
        and <code class="computeroutput"><span class="identifier">co_await</span></code> of an expired
        noexcept coro will cause undefined behaviour.
    </p>
    <h6>
        <a name="asio.overview.composition.coro.h2"></a>
        <span><a name="asio.overview.composition.coro.return"></a></span><a class="link"
                                                                            href="coro.html#asio.overview.composition.coro.return">Return</a>
    </h6>
    <p>
        A coro can also define a type that can be used with <code class="computeroutput"><span class="identifier">co_return</span></code>:
    </p>
    <pre class="programlisting"><span class="identifier">coro</span><span class="special">&lt;</span><span
            class="keyword">void</span><span class="special">()</span> <span class="keyword">noexcept</span><span
            class="special">,</span> <span class="keyword">int</span><span class="special">&gt;</span> <span
            class="identifier">c</span><span class="special">(</span><span
            class="identifier">any_io_executor</span><span class="special">)</span>
<span class="special">{</span>
  <span class="identifier">co_return</span> <span class="number">42</span><span class="special">;</span>
<span class="special">}</span>
</pre>
    <p>
        A coro can have both a <code class="computeroutput"><span class="identifier">Yield</span></code>
        and <code class="computeroutput"><span class="identifier">Return</span></code> that are non
        void at the same time.
    </p>
    <h6>
        <a name="asio.overview.composition.coro.h3"></a>
        <span><a name="asio.overview.composition.coro.result"></a></span><a class="link"
                                                                            href="coro.html#asio.overview.composition.coro.result">Result</a>
    </h6>
    <p>
        The result type of a coroutine is dermined by both <code class="computeroutput"><span
            class="identifier">Yield</span></code>
        and <code class="computeroutput"><span class="identifier">Return</span></code>. Note that in
        the follwing table only the yield output value is considered, i.e. <code class="computeroutput"><span
            class="identifier">T</span><span class="special">(</span><span class="identifier">U</span><span
            class="special">)</span></code> means <code class="computeroutput"><span class="identifier">T</span></code>.
    </p>
    <div class="table">
        <a name="asio.overview.composition.coro.result_type"></a>
        <p class="title"><b>Table 2. Result type deduction</b></p>
        <div class="table-contents">
            <table class="table" summary="Result type deduction">
                <colgroup>
                    <col>
                    <col>
                    <col>
                    <col>
                    <col>
                </colgroup>
                <thead>
                <tr>
                    <th>
                        <p>
                            Yield
                        </p>
                    </th>
                    <th>
                        <p>
                            Return
                        </p>
                    </th>
                    <th>
                        <p>
                            <code class="computeroutput"><span class="keyword">noexcept</span></code>
                        </p>
                    </th>
                    <th>
                        <p>
                            <code class="computeroutput"><span class="identifier">result_type</span></code>
                        </p>
                    </th>
                    <th>
                        <p>
                            <code class="computeroutput"><span class="identifier">completion_signature</span></code>
                        </p>
                    </th>
                </tr>
                </thead>
                <tbody>
                <tr>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="identifier">T</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="identifier">U</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="keyword">false</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="identifier">variant</span><span class="special">&lt;</span><span
                                    class="identifier">T</span><span class="special">,</span>
                                <span class="identifier">U</span><span class="special">&gt;</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="keyword">void</span><span class="special">(</span><span
                                    class="identifier">std</span><span class="special">::</span><span
                                    class="identifier">exception_ptr</span><span class="special">,</span>
                                <span class="identifier">variant</span><span class="special">&lt;</span><span
                                        class="identifier">T</span><span class="special">,</span>
                                <span class="identifier">U</span><span class="special">&gt;)</span></code>
                        </p>
                    </td>
                </tr>
                <tr>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="identifier">T</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="identifier">U</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="keyword">true</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="identifier">variant</span><span class="special">&lt;</span><span
                                    class="identifier">T</span><span class="special">,</span>
                                <span class="identifier">U</span><span class="special">&gt;</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="keyword">void</span><span class="special">(</span><span
                                    class="identifier">variant</span><span class="special">&lt;</span><span
                                    class="identifier">T</span><span class="special">,</span>
                                <span class="identifier">U</span><span class="special">&gt;)</span></code>
                        </p>
                    </td>
                </tr>
                <tr>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="identifier">T</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="keyword">void</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="keyword">false</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span
                                    class="identifier">T</span><span class="special">&gt;</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="keyword">void</span><span class="special">(</span><span
                                    class="identifier">std</span><span class="special">::</span><span
                                    class="identifier">exception_ptr</span><span class="special">,</span>
                                <span class="identifier">optional</span><span class="special">&lt;</span><span
                                        class="identifier">T</span><span class="special">&gt;)</span></code>
                        </p>
                    </td>
                </tr>
                <tr>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="identifier">T</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="keyword">void</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="keyword">true</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span
                                    class="identifier">T</span><span class="special">&gt;</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="keyword">void</span><span class="special">(</span><span
                                    class="identifier">optional</span><span class="special">&lt;</span><span
                                    class="identifier">T</span><span class="special">&gt;)</span></code>
                        </p>
                    </td>
                </tr>
                <tr>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="keyword">void</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="keyword">void</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="keyword">false</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span
                                    class="identifier">T</span><span class="special">&gt;</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="keyword">void</span><span class="special">(</span><span
                                    class="identifier">std</span><span class="special">::</span><span
                                    class="identifier">exception_ptr</span><span class="special">)</span></code>
                        </p>
                    </td>
                </tr>
                <tr>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="keyword">void</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="keyword">void</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="keyword">true</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span
                                    class="identifier">T</span><span class="special">&gt;</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="keyword">void</span><span
                                    class="special">()</span></code>
                        </p>
                    </td>
                </tr>
                <tr>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="keyword">void</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="identifier">T</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="keyword">false</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span
                                    class="identifier">T</span><span class="special">&gt;</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="keyword">void</span><span class="special">(</span><span
                                    class="identifier">std</span><span class="special">::</span><span
                                    class="identifier">exception_ptr</span><span class="special">,</span>
                                <span class="identifier">T</span><span class="special">)</span></code>
                        </p>
                    </td>
                </tr>
                <tr>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="keyword">void</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="identifier">T</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="keyword">true</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span
                                    class="identifier">T</span><span class="special">&gt;</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="keyword">void</span><span class="special">(</span><span
                                    class="identifier">T</span><span class="special">)</span></code>
                        </p>
                    </td>
                </tr>
                </tbody>
            </table>
        </div>
    </div>
    <br class="table-break"><h6>
    <a name="asio.overview.composition.coro.h4"></a>
    <span><a name="asio.overview.composition.coro.executor"></a></span><a class="link"
                                                                          href="coro.html#asio.overview.composition.coro.executor">Executor</a>
</h6>
    <p>
        Every coroutine needs to have its own executor. Since the coroutine gets
        called multiple times, it cannot take the executor from the caller like
        an <code class="computeroutput"><span class="identifier">awaitable</span></code>. Therefore
        a <code class="computeroutput"><span class="identifier">coro</span></code> requires to get
        an executor or an execution_context passed in as the first parameter.
    </p>
    <pre class="programlisting"><span class="identifier">coro</span><span class="special">&lt;</span><span
            class="keyword">int</span><span class="special">&gt;</span> <span
            class="identifier">with_executor</span><span class="special">(</span><span class="identifier">any_io_executor</span><span
            class="special">);</span>
<span class="identifier">coro</span><span class="special">&lt;</span><span class="keyword">int</span><span
                class="special">&gt;</span> <span class="identifier">with_context</span><span
                class="special">(</span><span class="identifier">io_context</span> <span class="special">&amp;);</span>
</pre>
    <p>
        It is to note, that an execution_context is defined as loosely as possible.
        An execution context is any object that has a <code class="computeroutput"><span
            class="identifier">get_executor</span><span class="special">()</span></code> function, which returns an
        executor that
        can be transformed into the executor_type of the coroutine. This allows
        most io_objects to be used as the source of the executor:
    </p>
    <pre class="programlisting"><span class="identifier">coro</span><span class="special">&lt;</span><span
            class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">with_socket</span><span
            class="special">(</span><span class="identifier">tcp</span><span class="special">::</span><span
            class="identifier">socket</span><span class="special">);</span>
</pre>
    <p>
        Additionally, a <code class="computeroutput"><span class="identifier">coro</span></code> that
        is a member function will check the <code class="computeroutput"><span class="keyword">this</span></code>
        pointer as well, either if it's an executor or an execution context:
    </p>
    <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">my_io_object</span>
<span class="special">{</span>
  <span class="identifier">any_io_executor</span> <span class="identifier">get_executor</span><span
                class="special">();</span>

  <span class="identifier">coro</span><span class="special">&lt;</span><span class="keyword">int</span><span
                class="special">&gt;</span> <span class="identifier">my_coro</span><span class="special">();</span>
<span class="special">};</span>
</pre>
    <p>
        Finally, a member coro can be given an explicit executor or execution context,
        to override the one of the object:
    </p>
    <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">my_io_object</span>
<span class="special">{</span>
  <span class="identifier">any_io_executor</span> <span class="identifier">get_executor</span><span
                class="special">();</span>

  <span class="identifier">coro</span><span class="special">&lt;</span><span class="keyword">int</span><span
                class="special">&gt;</span> <span class="identifier">my_coro</span><span class="special">(</span><span
                class="identifier">any_io_executor</span> <span class="identifier">exec</span><span
                class="special">);</span> <span class="comment">// it will use exec</span>
<span class="special">};</span>
</pre>
    <h6>
        <a name="asio.overview.composition.coro.h5"></a>
        <span><a
                name="asio.overview.composition.coro._code__phrase_role__identifier__co_await__phrase___code_"></a></span><a
            class="link"
            href="coro.html#asio.overview.composition.coro._code__phrase_role__identifier__co_await__phrase___code_"><code
            class="computeroutput"><span class="identifier">co_await</span></code></a>
    </h6>
    <p>
        The @c co_await within a <code class="computeroutput"><span class="identifier">coro</span></code>
        is not the same as <code class="computeroutput"><span class="identifier">async_resume</span><span
            class="special">(</span><span class="identifier">use_coro</span><span class="special">)</span></code>,
        unless both coros use different executors.
        If they use the same, the <code class="computeroutput"><span class="identifier">coro</span></code>
        will direclty suspend and resume the executor, without any usage of the
        executor.
    </p>
    <p>
        <code class="computeroutput"><span class="identifier">co_await</span> <span
                class="identifier">this_coro</span><span class="special">::</span></code> behaves the same as coroutines
        that use
        @c asio::awaitable.
    </p>
    <h6>
        <a name="asio.overview.composition.coro.h6"></a>
        <span><a name="asio.overview.composition.coro.integrating_with_awaitable"></a></span><a class="link"
                                                                                                href="coro.html#asio.overview.composition.coro.integrating_with_awaitable">Integrating
        with awaitable</a>
    </h6>
    <p>
        As the <code class="computeroutput"><span class="identifier">coro</span></code> member function
        <code class="computeroutput"><span class="identifier">async_resume</span></code> is an asynchronous
        operation, it may also be used in conjunction with <code class="computeroutput"><span class="identifier">awaitable</span></code>
        coroutines in a single control flow. For example:
    </p>
    <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span
            class="identifier">asio</span><span class="special">.</span><span class="identifier">hpp</span><span
            class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">asio</span><span
                class="special">/</span><span class="identifier">experimental</span><span class="special">/</span><span
                class="identifier">coro</span><span class="special">.</span><span class="identifier">hpp</span><span
                class="special">&gt;</span>

<span class="keyword">using</span> <span class="identifier">asio</span><span class="special">::</span><span
                class="identifier">ip</span><span class="special">::</span><span class="identifier">tcp</span><span
                class="special">;</span>

<span class="identifier">asio</span><span class="special">::</span><span class="identifier">experimental</span><span
                class="special">::</span><span class="identifier">coro</span><span class="special">&lt;</span><span
                class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span
                class="special">&gt;</span> <span class="identifier">reader</span><span class="special">(</span><span
                class="identifier">tcp</span><span class="special">::</span><span class="identifier">socket</span><span
                class="special">&amp;</span> <span class="identifier">sock</span><span class="special">)</span>
<span class="special">{</span>
  <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span
                class="identifier">buf</span><span class="special">;</span>
  <span class="keyword">while</span> <span class="special">(</span><span class="identifier">sock</span><span
                class="special">.</span><span class="identifier">is_open</span><span class="special">())</span>
  <span class="special">{</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span
                class="identifier">n</span> <span class="special">=</span> <span
                class="identifier">co_await</span> <span class="identifier">asio</span><span
                class="special">::</span><span class="identifier">async_read_until</span><span class="special">(</span>
        <span class="identifier">sock</span><span class="special">,</span> <span class="identifier">asio</span><span
                class="special">::</span><span class="identifier">dynamic_buffer</span><span
                class="special">(</span><span class="identifier">buf</span><span class="special">),</span> <span
                class="char">'\n'</span><span class="special">,</span>
        <span class="identifier">asio</span><span class="special">::</span><span
                class="identifier">experimental</span><span class="special">::</span><span
                class="identifier">use_coro</span><span class="special">);</span>
    <span class="identifier">co_yield</span> <span class="identifier">buf</span><span class="special">.</span><span
                class="identifier">substr</span><span class="special">(</span><span class="number">0</span><span
                class="special">,</span> <span class="identifier">n</span><span class="special">);</span>
    <span class="identifier">buf</span><span class="special">.</span><span class="identifier">erase</span><span
                class="special">(</span><span class="number">0</span><span class="special">,</span> <span
                class="identifier">n</span><span class="special">);</span>
  <span class="special">}</span>
<span class="special">}</span>

<span class="identifier">asio</span><span class="special">::</span><span class="identifier">awaitable</span><span
                class="special">&lt;</span><span class="keyword">void</span><span class="special">&gt;</span> <span
                class="identifier">consumer</span><span class="special">(</span><span class="identifier">tcp</span><span
                class="special">::</span><span class="identifier">socket</span> <span
                class="identifier">sock</span><span class="special">)</span>
<span class="special">{</span>
  <span class="keyword">auto</span> <span class="identifier">r</span> <span class="special">=</span> <span
                class="identifier">reader</span><span class="special">(</span><span class="identifier">sock</span><span
                class="special">);</span>
  <span class="keyword">auto</span> <span class="identifier">msg1</span> <span class="special">=</span> <span
                class="identifier">co_await</span> <span class="identifier">r</span><span class="special">.</span><span
                class="identifier">async_resume</span><span class="special">(</span><span class="identifier">asio</span><span
                class="special">::</span><span class="identifier">use_awaitable</span><span class="special">);</span>
  <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span
                class="special">&lt;&lt;</span> <span class="string">"Message 1: "</span> <span
                class="special">&lt;&lt;</span> <span class="identifier">msg1</span><span class="special">.</span><span
                class="identifier">value_or</span><span class="special">(</span><span class="string">"\n"</span><span
                class="special">);</span>
  <span class="keyword">auto</span> <span class="identifier">msg2</span> <span class="special">=</span> <span
                class="identifier">co_await</span> <span class="identifier">r</span><span class="special">.</span><span
                class="identifier">async_resume</span><span class="special">(</span><span class="identifier">asio</span><span
                class="special">::</span><span class="identifier">use_awaitable</span><span class="special">);</span>
  <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span
                class="special">&lt;&lt;</span> <span class="string">"Message 2: "</span> <span
                class="special">&lt;&lt;</span> <span class="identifier">msg2</span><span class="special">.</span><span
                class="identifier">value_or</span><span class="special">(</span><span class="string">"\n"</span><span
                class="special">);</span>
<span class="special">}</span>

<span class="identifier">asio</span><span class="special">::</span><span class="identifier">awaitable</span><span
                class="special">&lt;</span><span class="keyword">void</span><span class="special">&gt;</span> <span
                class="identifier">listen</span><span class="special">(</span><span class="identifier">tcp</span><span
                class="special">::</span><span class="identifier">acceptor</span><span
                class="special">&amp;</span> <span class="identifier">acceptor</span><span class="special">)</span>
<span class="special">{</span>
  <span class="keyword">for</span> <span class="special">(;;)</span>
  <span class="special">{</span>
    <span class="identifier">co_spawn</span><span class="special">(</span>
        <span class="identifier">acceptor</span><span class="special">.</span><span
                class="identifier">get_executor</span><span class="special">(),</span>
        <span class="identifier">consumer</span><span class="special">(</span><span
                class="identifier">co_await</span> <span class="identifier">acceptor</span><span
                class="special">.</span><span class="identifier">async_accept</span><span class="special">(</span><span
                class="identifier">asio</span><span class="special">::</span><span
                class="identifier">use_awaitable</span><span class="special">)),</span>
        <span class="identifier">asio</span><span class="special">::</span><span class="identifier">detached</span><span
                class="special">);</span>
  <span class="special">}</span>
<span class="special">}</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
  <span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_context</span> <span
                class="identifier">ctx</span><span class="special">;</span>
  <span class="identifier">tcp</span><span class="special">::</span><span class="identifier">acceptor</span> <span
                class="identifier">acceptor</span><span class="special">(</span><span class="identifier">ctx</span><span
                class="special">,</span> <span class="special">{</span><span class="identifier">tcp</span><span
                class="special">::</span><span class="identifier">v4</span><span class="special">(),</span> <span
                class="number">54321</span><span class="special">});</span>
  <span class="identifier">co_spawn</span><span class="special">(</span><span class="identifier">ctx</span><span
                class="special">,</span> <span class="identifier">listen</span><span class="special">(</span><span
                class="identifier">acceptor</span><span class="special">),</span> <span
                class="identifier">asio</span><span class="special">::</span><span
                class="identifier">detached</span><span class="special">);</span>
  <span class="identifier">ctx</span><span class="special">.</span><span class="identifier">run</span><span
                class="special">();</span>
<span class="special">}</span>
</pre>
    <h6>
        <a name="asio.overview.composition.coro.h7"></a>
        <span><a name="asio.overview.composition.coro.see_also"></a></span><a class="link"
                                                                              href="coro.html#asio.overview.composition.coro.see_also">See
        Also</a>
    </h6>
    <p>
        <a class="link" href="../../reference/co_spawn.html" title="co_spawn">co_spawn</a>, <a class="link"
                                                                                               href="../../reference/experimental__coro.html"
                                                                                               title="experimental::coro">experimental::coro</a>,
        <a class="link" href="cpp20_coroutines.html" title="C++20 Coroutines Support">C++20 Coroutines</a>,
        <a class="link" href="spawn.html" title="Stackful Coroutines">Stackful Coroutines</a>,
        <a class="link" href="coroutine.html" title="Stackless Coroutines">Stackless Coroutines</a>.
    </p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%">
    <tr>
        <td align="left"></td>
        <td align="right">
            <div class="copyright-footer">Copyright © 2003-2022 Christopher M. Kohlhoff<p>
                Distributed under the Boost Software License, Version 1.0. (See accompanying
                file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
            </p>
            </div>
        </td>
    </tr>
</table>
<hr>
<div class="spirit-nav">
    <a accesskey="p" href="cpp20_coroutines.html"><img src="../../../prev.png" alt="Prev"></a><a accesskey="u"
                                                                                                 href="../composition.html"><img
        src="../../../up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../home.png"
                                                                                            alt="Home"></a><a
        accesskey="n" href="../networking.html"><img src="../../../next.png" alt="Next"></a>
</div>
</body>
</html>
